I finally got around to looking up “priority date” – if I’m not mistaken, it looks like it’s just a sleazy way to head off “prior art” from (in this case) up to 11 years before the actual filing date for the patent (in 1997). If someone published this exact technique somewhere in 1992, for example, it wouldn’t count as “prior art” despite the fact that the filing date for the patent wasn’t even until half a decade later.

(I’m guessing that it may have started with an older, related audio processing technique that someone figured “hey, we can re-patent this for MP3 use and keep charging people for it!”, rather than outright fraud or something).

Pretty sure it still means the patent remains an implied threat until 2017 (but not totally positive).